1. Description of Prior Art
Caller Identity Delivery on Call Waiting (CIDCW) is an optional service presently offered to telephone subscribers by some Local Exchange Carriers (LECs) over conventional tip/ring lines. The service is useful in situations where telephonic communications are in progress between a first telephonic device and a second telephonic device, and a third party directs an incoming call to the first telephonic device. If this first telephonic device is connected to a tip/ring line that is activated to implement the CIDCW service, the LEC sends an alerting signal to the first telephonic device to alert the telephone subscriber as to the existence of an incoming call from a third party. If the first telephonic device responds to the alerting signal with an acknowledgment signal, then the LEC sends an identity signal (also referred to as a CID signal) over the tip/ring line to the first telephonic device. The identity signal is indicative of the identity of the third party. In the present example, the alerting signal and the identity signal together comprise a first alerting signal sequence. If the third party call is not answered within approximately ten seconds after the termination of the first alerting signal sequence, the LEC sends a second signal alerting sequence to the first telephonic device. This second alerting sequence includes an alerting signal and, provided that the first telephonic device responds to the alerting signal with an acknowledgment signal, the second alerting sequence also includes an identity signal. Although the present example describes an alerting signal sequence that includes both an alerting signal and an identity signal, it is to be understood that an alerting signal sequence may include any desired combination of alerting signals and/or identity signals.
The CIDCW service differs from conventional Call Waiting services in that CIDCW delivers the identity signal immediately after the alerting signal is delivered. This process provides telephone subscribers with convenience of identifying important incoming telephone calls while an existing telephone call is already in progress. In this manner, telephone subscribers are provided with information helpful in deciding whether or not to switch to the new, incoming telephone call from an existing telephone call. CIDCW may be considered an enhancement of conventional caller identification (caller ID) services, which deliver information pertaining to the identity of the calling party only when the called party is in an on-hook state.
Technologies in voice band data communications have matured to the point that modem usage is now fairly commonplace. For example, modems enable residence-based personal computing devices to access large mainframe computers at central business locations over conventional PSTN (public switched telephone network) telephone lines, and to access various on-line database services to retrieve various types of information. These applications often utilize a telephone connection for a rather lengthy period of time. It is not at all unusual for a telephone line to be in use for hours at a time while a telephone subscriber is performing on-line database searches. If a third party attempts to call the telephone subscriber while the subscriber is already engaged in modem-to-modem communications, the incoming caller will receive a frustrating busy signal, with no indication as to why the called party's line is busy.
The CIDCW service provides a potential solution to the above problem. Unfortunately, existing modem products and applications (modem software) are not compatible with the CIDCW service. Upon receipt of an incoming call from a third party, the CIDCW service sends an alerting sequence to the first telephonic device, which effectively blocks all modem-to-modem communications for a block-out period lasting on the order of a few seconds. Modems respond to such a block-out period by disconnecting from the tip/ring line, effectively terminating modem-to-modem communications, even after the CIDCW interruption has ended. The maximum block-out period is 2.6 seconds if the first telephonic device acknowledges receipt of the alerting signal with an acknowledgment signal, and 1.17 seconds if the first telephonic device does not acknowledge receipt of the alerting signal with an acknowledgment signal. In either case, this block-out period would be of a duration sufficient to cause termination of modem-to-modem communications unless special precautions are taken, as will be described in greater detail hereinafter.
After modem-to-modem communications have been interrupted due to the occurrence of one or more CIDCW alerting signal sequences, the modems attempt a retraining procedure to reestablish communications. Such retraining procedures typically require at least ten seconds. Due to the fact that a second CIDCW alerting sequence may be sent out approximately 10 seconds after termination of the first sequence (as described above), this second CIDCW alerting sequence will likely interfere with a modem-to-modem retraining procedure already in progress. Note that the performance of a modem-to-modem retrain procedure requires more than 10 seconds, and that retrain commences after the first CIDCW alerting sequence has terminated, i.e., after the FSK data block has been received at the modem. Consequently, the second CIDCW alerting sequence proves to be destructive in that it arrives in the middle of retrain recovery.
What is needed is a technique for providing interoperability between a first modem and a CIDCW service. Such a technique should enable the first modem to receive a caller identity (CID) signal without dropping an existing connection between the first modem and a remote modem. The first modem should be equipped to implement this technique independent of the specific capabilities of the remote modem, so that these techniques can be utilized in a system that may include an existing modem product not constructed in accordance with the embodiments disclosed herein.